where \(\operatorname{tr}\)  is Olver's trace notation defined by [1]}
\(\operatorname{tr}F_1(\mathbf {x}^{(1)})F_2(\mathbf {x}^{(2)})F_3(\mathbf {x}^{(3)})=F_1(\mathbf {x})F_2(\mathbf {x})F_3(\mathbf {x}),\) 
